In order to meet wireless data traffic demands that have increased since 4th Generation (4G) communication system commercialization, efforts are being made to develop an improved fifth-generation (5G) communication system or a pre-5G communication system. For this reason, the 5G communication system or the pre-5G communication system is called a beyond 4G network communication system or a post LTE system.
In order to achieve a high data transmission rate, an implementation of the 5G communication system in an mmWave band (for example, 60 GHz band) is being considered. To mitigate the path loss of a radio wave and increase the transmission distance of a radio wave in the mmWave band, technologies such as beamforming, massive MIMO, Full Dimensional MIMO (FD-MIMO), an array antenna, analog beam-forming, and a large scale antenna are under discussion for the 5G communication system.
Further, technologies such as an evolved small cell, an advanced small cell, a cloud Radio Access Network (cloud RAN), an ultra-dense network, Device to Device communication (D2D), a wireless backhaul, a moving network, cooperative communication, Coordinated Multi-Points (CoMP), and interference cancellation have been developed to improve the system network in the 5G communication system.
In addition, in the 5G system, Advanced Coding Modulation (ACM) schemes such as Hybrid FSK and QAM Modulation (FQAM) and Sliding Window Superposition Coding (SWSC), and advanced access technologies such as Filter Bank Multi Carrier (FBMC), Non Orthogonal Multiple Access (NOMA), and Sparse Code Multiple Access (SCMA) have been developed.
A description of a general D2D discovery operation scenario is as follow.
In an LTE system, sidelink transmission (known as Device to Device transmission or D2D transmission) may be performed in an uplink frequency spectrum (in the case of a frequency division duplex (FDD) or an uplink subframe (in the case of a time division duplex (TDD)).
The sidelink transmission is restricted to a subset of an uplink resource, i.e., a subset of a subframe in a time domain or a subset of a resource block (RB) in a frequency domain.
The sidelink transmission may provide two types of proximity based services (ProSe), i.e., direct discovery and direct communication.
FIG. 1 illustrates a scenario in which UEs inside two neighboring cells discover each other.
In ProSe discovery, mutually neighboring UEs may discover each other. As illustrated in FIG. 1, a UE-4 100 in a first cell 110 may discover a UE-5 102 in the identical cell as well as a UE-3 104 in a neighboring cell 120.
Each UE may transmit a discovery message by using a discovery resource in a discovery resource pool configured by a network, and may receive a discovery message transmitted from another UE in the identical cell or a neighboring cell.
Two types of discovery procedures may be defined according to how a resource is allocated. A first type of discovery procedure is a procedure in which each UE selects a discovery resource on the basis of a rule predefined for the each UE. A second type of discovery procedure is a procedure in which a resource for discovery message transmission is allocated to each UE by a base station (eNB).
Synchronization is a prerequisite for the sidelink transmission. In order to make synchronization between UEs possible, each eNB may configure some synchronization resources for the transmission of Sidelink Synchronization Signal (SLSS) or a Sidelink Synchronization Sequence (SSS) on a fixed period (e.g. 40 ms) basis, and may configure some indispensible system information.
Each cell has an SLSS specific to itself, and the SLSS may include a primary SLSS and a secondary SLSS.
The eNB may instruct a UE to transmit an SLSS, and a UE, which satisfies a predefined triggering condition, may transmit an SLSS.
The transmitted SLSS may be used to acquire time and frequency synchronization for sidelink transmission or reception by a UE. Further, SLSS transmission in one cell may allow UEs in a neighboring cell to be synchronized with the one cell so that the UEs discover each other.
System information for sidelink transmission may be derived from a parameter, which is signaled from an eNB, and may be derived from a pre-configured parameter.
In the above-described D2D discovery, the following item becomes an issue.
Even when network coverage is not available, for example, at the time of the occurrence of network failure, at the time of the occurrence of attenuation due to a local environment, or at the time of the occurrence of simple lack of coverage, ProSe discovery maintenance capacity should be guaranteed. In particular, the ProSe discovery maintenance capacity is required to be guaranteed for the purpose of public safety.
FIG. 2 illustrates D2D discovery in an out-of-coverage scenario and a partial coverage scenario.
In FIG. 2, when a network (i.e. eNB 200) is not available, it is required to consider a method for enabling discovery between out-of-coverage (OOC) UEs (e.g. UE-1 202 and UE-2 204) and a method for enabling discovery between an out-of-coverage UE (e.g. UE-3 206) and an in-coverage (IC) UE (e.g. UE-4 212 or UE-5 214) (i.e., partial coverage scenario).